1. Field of the Invention
The present invention relates to a method of manufacturing a substrate for a liquid discharge head used for a liquid discharge head.
2. Description of the Related Art
As a liquid discharge head for discharging liquid, an ink jet head for discharging ink as the liquid is known. In a typical ink jet head, a through opening (ink supply opening) is provided in a substrate having ink discharge pressure generating elements formed thereon, and the ink is supplied from a surface opposite to a surface having the ink discharge pressure generating elements formed thereon.
As a method of forming an ink supply opening of this type, U.S. Pat. No. 6,143,190 proposes a method in which a silicon substrate with a patterned mask material is anisotropically etched in a strong alkaline solution. The gist of this method is to form an ink supply opening, the inner walls of which are all (111) surfaces, in a silicon substrate by making (111) surfaces having a low etching rate emerge utilizing anisotropic etching of silicon. When anisotropic etching is performed on a silicon (001) substrate, (111) surfaces are formed at an angle of 54.7 degrees inward from an end of an opening in the mask, and hence the opening width of the through opening can be defined by the width of the mask formed on a back surface of the silicon substrate.
However, in order to form the through opening, this method requires a large opening width in the mask on the back surface. The opening width of the mask on the back surface is defined by the thickness of a wafer and the opening width on a front surface of the wafer. For example, when the thickness of the silicon substrate is 625 μm and the opening width on the front surface is 120 μm, the opening width in the mask on the back surface has to be 1000 μm. Thus, in a method of forming an ink supply opening of this kind, the size of the chip substrate is defined by the size of the ink supply opening, and hence miniaturization of the chip substrate is limited.
On the other hand, U.S. Pat. No. 6,648,454 discloses a manufacturing method which achieves miniaturization by suppressing formation of a (111) surface through combination of deep digging of silicon and anisotropic etching.
This manufacturing method includes the steps of deeply digging silicon to form a blind trench (blind hole) in a portion where an ink supply opening is to be formed, and thereafter, performing anisotropic etching. By forming the trench, time necessary for the anisotropic etching to form a through opening is shortened, and a miniaturized chip in which the width of the ink supply opening is small is manufactured.
As disclosed in U.S. Pat. No. 6,648,454, by forming a trench to decrease the amount of Si etched by the anisotropic etching, the width of the ink supply opening in a transverse direction can be made small, and hence a miniaturized chip can be manufactured. In a simplified manner, an opening width X of the ink supply opening is determined by a width L of the trench and an amount E of the etched Si, and the relational expression can be calculated as X=L+2E.
In this case, as the formed trench is deeper, the amount of the etched Si becomes smaller for forming the through opening, and hence the miniaturization can be further enhanced. For example, when a trench is formed at a width of 200 μm and at a depth of 500 μm in an Si wafer at a thickness of 625 μm, the width X of the ink supply opening is calculated to be 450 μm. Meanwhile, when the depth is 550 μm, X can be as small as 350 μm.
However, with regard to a substrate for an ink jet head in which, after wiring and nozzles are formed on a surface of the substrate, the blind trench is formed and anisotropic etching is performed from a back surface of the substrate to form the through opening, the following problem arises. Specifically, taking into consideration damage of the substrate and stability of the depth of the processing, it is difficult to deeply dig Si with accuracy, and a miniaturized ink supply opening cannot be manufactured.
In U.S. Pat. No. 6,648,454, sandblasting, dicing, dry etching, and laser processing are listed as exemplary methods of processing Si, but those methods cannot be adapted for deeply digging Si in manufacturing the above-mentioned substrate for an ink jet head for the following reasons.
First, machining such as sandblasting or dicing generates various sizes of cracks, which causes more chipping and cracks of the substrate. Second, dry etching has a low etching rate, and hence is less productive and is not a realistic method.
With regard to laser processing, Si removed in the processing (hereinafter, referred to as debris) accumulates at the bottom of and on an end face of the processed hole to cause scattering of an entering laser, which makes it difficult to deeply dig Si. If the laser spot diameter is increased to make the processed area larger, the debris is more easily let out from the processed hole. However, in this case, the increased spot diameter decreases irradiated energy per unit area, and hence the processing ability is lowered. Further, a high-power laser can deeply dig Si. However, since the processing energy is too high, it becomes considerably difficult to control the depth of the processed hole. Further, in deeply digging Si, the hole may pierce the Si substrate. In that case, a further problem arises that wiring and nozzles already formed on the surface of the substrate are damaged to spoil the function as an ink jet head.